Block copolymer micelles and vesicles are mostly prepared by the solvent mixing method, where the block copolymer is first dissolved in a common solvent for both blocks, which is then mixed with a selective solvent, mostly water, to induce self-assembly into the desired structure. Using a combination of microfluidic flow-focusing and capillary interdiffusion experiments combined with in-situ small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM) we investigated the structural evolution during solvent mixing from single block copolymers into spherical and cylindrical micellar intermediate structures into vesicles. We find that micelle formation is very fast and diffusion-limited, occurring on time scales of a few milliseconds. The development of an ordered lyotropic micellar phase is completed within 1 s. The structural transformation into cylindrical micelles occurs over several hours, which subsequently evolve into vesicles over time scales of days. Whereas the first two steps are transport-limited, the two latter processes involve large activation energies related to micellar fusion against the sterically stabilizing micellar coronas, which corresponds to much longer time scales of self-assembly. (C) 2016 Published by Elsevier Ltd.